"This shall be the law of the Metzora on the day of his purification: He shall be brought to the Kohen" (14:2). Rabbi Nison Alpert T'ZL(1927-1986) brings down the Gemara in Arachin 15b quoting Resh Lakish to elaborate on this Posuk.
"Resh Lakish said: "What is the meaning of that which is written, "This shall be the law of the Metzora"? It means: This shall be the law of the Motzi Shem Ra (defamer)." Rabbi Alpert T'ZL points out that "unjustified pride lies at the very heart of the sin of Motzi Shem Ra." Unlike other forms of Tumah (impurity) Rabbi Alpert stresses that the Tumah of the Metzora is strictly dependent upon the words of the Kohen.
The Dubno Maggid (1741-1804) also touches on this subject and says that the Kohen must make the pronouncement precisely because the Metzora claims that he was just saying mere words and everyone knows the expression "sticks and stones can break my bones but words can never hurt me". This is the problem with the Metzora. He under-appreciates and miscalculates how damaging mere words can be. It is specifically for this reason that the Kohen must use "mere words" to pronounce the Metzora,Tamei or Tahor. Mere words from the Kohen can make you an outcast or part of the community.
This is the power of Lashon HaRa. It destroys. As I have stated in Parshat Tzav quoting Gemara Arachin 15b, Lashon HaRa is worse than Idolatry, Murder, and Illicit Relations because it causes and brings about all three Cardinal Sins. The Kli Yakar says the word Metzora stands for "Motzi Ra". The sin that is hidden inside, now is manifest on the outside. The Tzaraat is the outward development of what is wrong on the inside.
The question that is raised is why there is no Tzaraat today? What happened to Tzaraat? Did it just disappear? In answer, I will start with the Halacha (VaYikra 13:12-13) that if Tzaraat covered the entire body of an individual, he is deemed Tahor. Rabbeinu Bachya(1263-1340) explains that this is unintuitive and is an example of a "Chok"-a mitzvah unintelligible to the limited human mind. The Netziv (1817-1893) explains the Posuk, "The Kohen shall look and behold!-the affliction has covered his entire flesh. Then he shall declare the affliction to be Tahor: having turned completely white, it is Tahor"(13:13) in a unique way. The Netziv says that the Metzora who is covered with Tzaraat from head to toe is too far gone to achieve the purpose of Tzaraat which is to make an individual do Tshuva (Repentance).
G-d only strikes an individual as he did Moshe Rabeinu's Sister Miriam (Bamidbar 12:1-16, and Devarim 24:9) in order to improve the person. G-d only "bothers" people he can change. He does not afflict people who have "graduated" and no longer feels He can change. This concept is not easy to understand but it is how the Netziv tries to explain why the Metzora with whole body Tzaraat is Tahor. It is better to make such an individual Tahor. The Tzaraat will not achieve the purpose it is designed to achieve in this individual. Unfortunately, this is the situation we live in today. Hence, there is no Tzaraat. Tzaraat will not achieve its purpose in the Age we live in. We are not on the level that Tzaraat will work to make us do Tshuva. G-d, essentially has declared us Tahor by default.
We are, unfortunately, on such a low level. Only Moshiach can save us as is so poignantly illustrated in this famous Braita which I will quote in its entirety because it is so powerful and apropos. In Gemara Sanhedrin 97a "Rabbi Yehuda says: In the generation when the son of David will come, the meeting place will be used for licentiousness, the Galilee will lay waste, and the Gavlan (some say northeastern Israel, others say southern Israel), the people of the border will wander from town to town and not be granted favor, the wisdom of the scholars will decay, those who fear sin will be despised, the face of the generation will be like the face of a dog (according to R'Yisrael Salanter the behavior of the Leaders of the generation will resemble that of dogs), truth will be absent as it is stated(Isaiah 59:15) "and the Truth will be absent, and he that turns away from evil, will become foolish".
People will eat, drink, and rejoice and Torah knowledge will return to its students. In the sixth year there will be sounds, in the seventh year there will be wars, in the aftermath of the seventh year, the son of David will come." The moral and spiritual decline which equates us with the Metzora who is completely covered by Tzaraat alludes to the Messianic Era when we will be pulled out of this degenerate state by Moshiach. This is the lesson, as I see it, of the Netziv. G-d will make us Tahor(pure) even though we might have reached the 49th level of Tumah. May the Moshiach come speedily in our day. Shabbat Shalom.